Concerning a gasoline engine, hazardous components in exhaust gas has been steadily decreased by strict regulations of exhaust gas and advance in technology that can deal with the regulations. However, concerning a diesel engine, there are still many technical problems left with purification of exhaust gas because the exhaust gas contains particulates (particulate matter: such as soot composed of carbon particulates or soluble organic fractions (SOF), hereinafter referred to as PM).
Accordingly, in recent years, the oxidation catalyst capable of oxidizing the PM, particularly soot components, at low temperature have been developed. For example, Japanese Unexamined Patent Application Publication No. 2004-42021 (Document 1) discloses a catalyst composition containing silver (Ag) and/or cobalt (Co) stabilized ceria (CeO2), which facilitates soot oxidation during the regeneration of DPF. Here, a molar ratio between Ag and ceria is preferably in a range from 4:1 to 1:4 (which corresponds to a content (mol %) of Ag relative to a total amount of Ce and Ag from 20 mol % to 80 mol %), and more preferably in a range from 3:1 to 1:3 (which corresponds to a content (mol %) of Ag relative to the total amount of Ce and Ag from 33 mol % to 67 mol %). Moreover, it is reported that a mixture having an Ag content at 75 mol % and a Ce content at 25 mol % has the highest degree of activity. Meanwhile, it is reported that a mixture having an Ag content at 25 mol % and a Ce content at 75 mol % is also active in soot oxidation even when oxygen is used as a sole oxidant. In this case, a reactive oxygen species can be produced in the gas phase. Here, a method of manufacturing the catalyst described in Document 1 is a method of obtaining the catalyst composition having a porosity wherein a pore size is centered around 70 to 200 Å and a high specific surface area ranging from 14 to 150 m2/g .
The catalyst composition is manufactured by impregnating a cellulose material (Whatman (registered trademark) Filter Paper 540) with a nitrate precursor and burning the cellulose at 600° C. and for two hours after drying the material overnight at a room temperature.
There are roughly two evaluation methods adopted in this Document 1. One is a method of measuring a decomposition rate of a substance obtained by subjecting diesel soot and the catalyst composition to loose-contact with a spatula by means of TGA under a 10 percent oxygen atmosphere, for example. The other is a method of conducting a pressure drop balance test by means of DPF. Moreover, in the TGA evaluation of the catalyst composition having an Ag content of 75 mol %, for example, an oxidation rate at 323° C. is equal to 0.117 hr−1 even in fine conditions of NO2: 1010 ppm and O2: 10%, and by use of the best catalyst composition. That is, a proportion of the soot to be oxidized in an hour is merely equal to 11.7% even under the condition where there is a sufficient amount of a powerful oxidant such as NO2. Meanwhile, in the pressure drop balance test, an Ag—Ce based catalyst composition CPF-15 is deemed to be capable of oxidizing most of the PM at a temperature around 325° C., but this test result contradicts the foregoing test result. Accordingly, the present inventors consider that soot components or the like slips through in an actual pressure drop balance test. Hence it was not possible to oxidize a carbon-containing component such as the soot and a component such as HC, CO or NO sufficiently at a low temperature by the catalyst compositions disclosed in Document 1, i.e. merely by presence of a set of Ag and CeO2 or a set of Co and CeO2.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2005-144402 (Document 2) discloses a partial oxidation catalyst of hydrocarbon having a core-shell structure wherein a core portion is formed by a catalytically active component made of particulates of a metal selected from iron group metals, and the periphery of the core portion is coated with a shell portion made of a catalyst support component selected from any of silica, alumina, zirconia, and titania.
Further, J. Colloid Int. Sci. 283 (2005) 392-396 (Document 3) discloses a method of coating Ag nanoparticles with silica by a sol-gel method called a Stober method.
However, it is hard to obtain activity of the components of the core portions in the partial oxidation catalyst having the catalytically active component that is strongly coated with the shell portion such as the catalyst disclosed in Document 2 and in the particles having the core-shell structure in which the capsular coating is formed by a condensation reaction as disclosed in Document 3. Accordingly, it was not possible to oxidize a carbon-containing component such as soot and a component such as HC, CO or NO sufficiently at a low temperature.